Angular tracker responsive to penetrating radiation

ABSTRACT

Disclosed is a system for deriving a direct digital indication of the position of a penetrating radiation source, e.g., a gammaray of X-ray source. A detector array having multiple channels with the same field of view is irradiated by the source. Each channel includes a different number of radiation-receiving areas to derive the digital indication. The detector array, in certain embodiments, is shielded with a slit plate or shadow mask. In another embodiment, a single multiple detector channel is shielded with a shadow mask and step changes in illumination of each detector drive a position indicating counter.

United States Patent [72] Inventor William B. Joyce Basking Ridge, NJ.121] Appl. No 637.214 {22] Filed May 9. 1967 [45] Patented Feb. 9, 1971[73 Assignee Industrial Nucleonics Corporation a corporation of Ohio[54] ANGULAR TRACKER RESPONSIVE TO PENETRATING RADIATION 22 Claims, 22Drawing Figs.

[52] U.S. Cl 250/833, 250/715, 250/836 250/105, 250/106 [51] Int. ClG01t 1/16 [50] Field of Search 250/833, 71.5, 83.6, 105, 203; 343/1 13[56] References Cited UNlTED STATES PATENTS 3,205,361 9/1965 Albus250/203 3,291,990 12/1966 Lentz 3,440,426 4/1969 BUSh ABSTRACT:Disclosed is a system for deriving a direct digital indication of theposition of a penetrating radiation source, e.g., a gamma-ray of X-raysource. A detector array having multiple channels with the same field ofview is irradiated by the source. Each channel includes a differentnumber of radiation-receiving areas to derive the digital indication.The detector array, in certain embodiments, is shielded with a slitplate or shadow mask. in another embodiment, a single multiple detectorchannel is shielded with a shadow mask and step changes in illuminationof each detector drive a position indicating counter.

iATENTEI] sea 919w SHEET 1 OF 6 WITHOUT PLATES 73 g FIGS WITH PLATES 73WITHOUT PLATES 73 WITH PLATES 13 WILLIAM B. JOYCE FIG] M07 ATTORNEYPATENTEBFEB'QIHII E r 3562528 SHEET 2 OF 6 4o f4| 42 49 IOIJ DET o DETIDET 2 DET9 CLOCK RANGE COMP BINARY LEVEL DET I 0 BINARY LEVEL DET 0 ssBINARY |o3 LEVEL DET- BINARY i o LEVEL DET I} 0} I02 H08 READOUT 42 [I06[I07 nos "2 0 RC DET INT GATE SCHM'DT 2 I TRIGGER H09 I RANGE comb 8|INVENTOR wuu/m a JOYCE ATTORNEY PATENTED FEB 9 ISYI SHEET 3 OF 6 T=TRANSMISSIVITY THROUGH THE SHIELD B BACKGROUND ma a INVENTOR WILLIAM B.JOYCE ATTORNEY P ATENTED FEB 9 IQYI SHEET & 0F 6 243 FIG I8 INVENTORWILLIAM B. JOYCE ATTORNEY PATENTFU m 919?:

"SHEET 6 BF 6 I FIGZI \\\\SOURCE 291, 292 293 v DETECTORS INT INT ESHOLDl TECTORS l 3064/ DIFF DIFF 1, 1' l H022 v 30s I FF 3 L J y su CONVERTERAND READOUT INVENTOR WILLIAM B. JOYCE ATTORNEY ANGULAR TRACKERRESPONSIVE TO PENETRATING RADIATION The present invention relatesgenerally to systems for locating objects carrying sources of radiationand more particularly 5 to a system for deriving a direct digitalindication of the position of a penetrating radiation source.

Systems for tracking the position of objects carrying radiation sourcesare known and described in the prior art by U.S. Pat Nos. 3,291,987;3,291,988; 3,291,989 and 3,291,990. Typically penetrating radiationsources emit X-rays and gamma-rays, whereby atmospheric conditions donot affect their propagating characteristics. In the presentspecification and claims, penetrating radiation is defined as radiationpenetrative of clouds and fog and incapable of being focused like lightand infrared radiations. These sources may or may not have a randomenergy distribution.

In the prior art systems, range is computed from the azimuth andelevation angle parameters derived in response to radiation impinging ona tracking detector from a penetrating radiation source carried on atracked object. Each angle parameter is synthesized by comparing theoutputs of a pair of detectors having differing fields of view. Eachdetector has a single channel output from which is derived a signalamplitude proportional to the level of the energy impinging thereon. Ifthe tracked object lies on a bisector between the detector pair, thesignals are of equal amplitude. As the object moves to one side or theother of the bisector the relative signal amplitudes from the detectorsare correspondingly varied.

While the prior art systems function admirably in tracking many targets,errors arise in tracking targets moving with a large angular velocityrelative to the detector. It can be shown mathematically that trackingan object at a range of 300 feet to within 1 minute of arc in a field of60 requires a random energy source with a count rate in excess of energyparticles per second. Presently known random energy, penetratingradiation sources that are capable of atmospheric penetration withoutexcessive attenuation as a function of range are incapable of such countrates.

if the count rate is lower than stated, noise, in the form of backgroundradiation, can mask the signal from a tracked radiation source at thestated angular resolution. For example, to track a target and provide asignal indicative of its angular position once every 0.] seconds with aresolution of 0.6 minutes of are, it can be shown that a detector mustreceive 10 energy counts per second. To determine how many of theseenergy counts or particles will reach a detector, it is necessary tocalculate the angle subtended by the detector in an omnidirectionalfield of the energy source. If the detector array has an area ofapproximately 1 square foot and is positioned at a range of 1,000 feetfrom the source, the detector subtends a solid angle that is 10- of thefield irradiated by the source. Hence, before considering the effects ofattenuation and detector efficiency, the maximum number of particlesimpinging on the detector at such a range is only l0, or a factor of 102 below the minimum required to attain the stated resolution.

According to one aspect of the present invention, 1 have found thatresolution to the stated angles and sampling times can be achieved byproviding a plurality of detector channels. With respect to someradiation sources, random energy distribution versus time functions canbe mathematically expressed in accordance with the Poisson distributionformula:

A k pun =eenergy from the radiation source, the number of counts thatmust be received by the entire array is reduced to 3l counts per secondper channel to achieve the resolution recited supra. In other words,increasing the number of channels from one to nine, reduces the requiredreceived count rate by a factor on the order of 10 .'This allows the useof a small strength source. 1

In several embodiments of the present invention, a detector assembly forachieving extremely high resolution with very short sampling timescomprises a plurality of detectors'each having a different'number ofdetecting areas and an identical field of view for the penetratingradiation source. Each detector channel feeds a binary decision circuit,whereby a multibit binary code indicative of the position of the randomenergy level versus time source isprovided without the intermediary ofan analog-to-digital converter. A coded signal is derived since eachcode combination is uniquely associated with aparticular position ofenergy impinging on the detector.

In one embodiment of the invention, each detector channel includes aplurality of cylindricallike detectors having arcuate shields placedbefore them. A different number of arcuate shields is placed before eachdetector to form the different number of detecting areas.

According to a further embodiment of the invention, the detectors arelinear and disposed in straight, parallel lines. Shielding each of theelongated detectors is a different number of planar shields to establishthe different number of detecting areas for each channel. To provideangular information indicative of the source being tracked, a slit plateis positioned between the source and the detector array. While theplanar embodiment of the invention is easier to construct than theembodiment utilizing arcuate shields and cylindrical detectors, the slitplate, in certain instances, does not adequately collimate energy fromthe source, whereby the possibility of several detector areas in thesame channel being illuminated arises.

To avoid ambiguity that might be introduced by failure of the slit plateto collimate adequately, still a further embodiment of the inventionincorporates a shadow shield between the source and a pluralityof planardetector channels. The shield in such embodiment is positioned so thatall ofthe detector channels are selectively illuminated from one side toa shadow edge defined by the shield.

Several different detector configurations with the shadow shields can beemployed. In one embodiment, each detector channel includes a singledetector with a plurality of shields, substantially the same as with theembodiment utilizing the slit plate. With the shadow detector, means isprovided to detect sudden changes in the energy impinging on thedetector, as occurs in response to movement'of the radiation source toan area where the detector is illuminated to a lesser or greater extend.

According to still another embodiment of the invention utilizing theshadow shield, the need for means detecting sudden changes in thedetector output is obviated by employing, in each detector channel, aplurality of different detectors. 1n the multiple detector per channelembodiment, each channel is divided into at least two groups ofdetectors, with a detector of the first group always being adjacent to adetector of the second group. Energy levels impinging on the detectorsin the first and second groups are compared to derive an indication ofthe angular position of the tracked source.

According to still a further aspect of the present invention, a directdigital readout is derived utilizing the shadow technique and a singlechannel of detectors that are divided into two groups. Energy levels ofthe radiation impinging on the first and second groups are compared anda counter, or other storage means, is activated in response to eachsudden change in the energy level impinging on one of the detectors.Sudden changes in energy level are sensed by comparing the outputs ofthe first and second groups in each channel. While the single channeldetector employing a shadow mask or shield has less resolution than theplural channel systems, it enables a direct resolution than the pluralchannel systems, it enables a direct digital signal indicative of sourcelocation to be derived with much less complexity than the plural channelsystems.

It is, accordingly, an object of the present invention to provide a newand improved system for tracking targets carrying sources of penetratingradiation not requiring high-source strengths.

It is an additional object of the present invention to provide a new andimproved system for tracking targets carrying penetrating radiationsources with greater resolution and higher sampling rates than prior artsystems.

Another object of the present invention is to provide a new and improvedsystem for tracking objects carrying penetrating radiation sources andsusceptible to movement at relatively large angular velocities, whichsystem includes a plural channel detector array responsive to radiationfrom said sources.

A further object of the present invention is to provide a system forderiving a digital signal indicative of the position of a penetratingradiation source without utilizing an analog-todigital converter.

Still another object of the present invention is to provide a new andimproved system adapted to detect the position of a source ofpenetrating radiation, i.e., radiation penetrative of clouds and fog andincapable of being focused like light and infrared radiations, withgreater accuracy and higher sampling rates than previously proposedsystems.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of several specific embodiments thereof,especially when taken in conjunction with the accompanying drawings,wherein:

FIG. 1 is a diagrammatic illustration of the environment in which thepresent invention is designed to be utilized; FIG. 2 is a schematicdrawing of one embodiment of a detector in accordance with the presentinvention;

FIG. 3 is a perspective view of the detector array schematicallyillustrated by FIG. 2;

FIG. 4 is a cross-sectional view illustrating a single detection areachannel of the FIG. 3 array;

FIG. 5 is a plot of amplitude versus position for the detector channelillustrated by FIG. 4;

FIG. 6 is a perspective view of a two detector area channel ofthe FIG. 3array;

FIG. 7 is a plot of amplitude versus position for the detector channelillustrated by FIG. 6;

FIG. 8 is a block diagram of the apparatus utilized for readinginformation from the array of FIG. 3;

FIG. 9 is a circuit diagram of one binary level detector, employed inthe circuit of FIG. 8;

FIGS. 10 and 11 are plots of the amplitude versus angular positionoutput of the detector channel including two detector areas, for sourcesat short and long ranges, respectively;

FIG. 12 is a perspective view of another embodiment of a single detectorchannel of the present invention;

FIG. 13 is a cross-sectional view of the detector channel of FIG. 12,taken through the lines 13-13;

FIG. 14 is a perspective view of a plurality of channels, such asillustrated by FIG. 12, in stacked relationship;

FIG. 15 is a perspective view ofa further embodiment ofthe presentinvention wherein a planar detector array is employed in combinationwith a shield having a slit;

FIG. 16 is a side view of the embodiment illustrated by FIG. 15;

FIG. 17 is a perspective view of still another embodiment of the presentinvention wherein a planar detector array and a shadow mask or shieldare employed;

FIG. 18 is a side view of the embodiment illustrated by FIG. 17;

FIG. 19 is a circuit diagram of the readout apparatus employed with theembodiment of FIG. 18;

FIG. 20 is a circuit diagram of the readout apparatus utilized with ashadow mask and a single planar detecting channel, of the typeillustrated by FIG. 17;

FIG. 21 is a perspective view of a modification of the systemillustrated by FIGS. 17 and I8; and

FIG. 22 is a circuit diagram of the readout apparatus util ized inconjunction with the system of FIG. 21.

Reference is now made to FIG. 1 of the drawings. wherein there isillustrated a tracking system for determining the position of movingaircraft 21, having a source 22 of penetrating radiation. In a typicalexample, source 22 may emit gammaor X-ray radiation, or any other knowntype of penetrative radiation. Source 22 is fixedly mounted on aircraft21, which is at a relatively low altitude above the ground and is movingat relatively large velocities. For example, the aircraft speed may beon the order of 400 mph and have an altitude of 500 feet.

Positioned on the ground at predetermined separated points, aredetectors 23, 24 and 25, adapted to receive radiation from source 22.Detectors 23-25 derive digital signals indicative of the azimuth andelevation of aircraft 21 relative to each of their positions and feed acomputer that tracks the movement of aircraft 21, in a manner similar tothat described in the aforementioned prior art patents. In response tothe digital signals from detectors 23-25, the computer derives a signalindicative of the aircraft height and ground position. In addition, thecomputer generates signals to drive detectors 23- 25 so that aircraft 21remains in the field of view of each of the detectors. Alternatively,the detector may be mounted in the moving object to enable computationof the angular position relative to a fixed radiation source.

Since the present invention is concerned only with an anglemeasuringdetector per se, for measuring one angle parameter, the followingdiscussion is given assuming that the elevation angle I 'of aircraft 21is measured from detector station 23. It is to be understood, however,that azimuth angles can be measured with substantiallythe sameequipment.

The principles of one embodiment of the present invention are bestcomprehended by reference to FIG. 2. In FIG. 2, detector array 23 isillustrated as comprising a circularly-shaped penetrating radiationdetector 31, which may comprise a scintillation counter, ionizationchamber, Geiger-Mueller tube, semiconductor, or other type ofpenetrative radiation detector. In either case, detector 31 derives anumber of pulses, the number of which is dependent upon the count rateof the radiant energy impinging thereon.

Except for approximately a 60 field of view that is defined by theradially extending edges 33 and 34, detector 31 is surrounded bypie-shaped radiation shielding 32, which may be lead or other material.Shield 32 effectively blocks substantially all environmental, backgroundradiation and prevents such radiation from reaching detector 31.

The arcuate 60 angular surface of detector 31 between radially extendingedges 33 and 34 of shield 32 is selectively exposed to radiation fromsource 22. Those portions of shield 32 exposed to radiation from source22 are determined by shield array 35, positioned between and extendingfrom edges 33 and 34. For purposes of simplicity and explanation, shieldarray 35 is illustrated as comprising four different annular sections36-39, having one, two, four and eight shielded areas, respectively.Shield areas 36-39, shown on the drawing with a hatched cross section,are separated by gaps or regions that enable radiation from source 22 tobe propagated to detector 31.

Each of sections 36-39 occupies a different longitudinal position alongthe axis of detector 31 and has a different detector area therebyassociated with it. In effect, detector 31 therefore comprises fourseparate detectors, one responsive to radiation transmitted throughsection 36, a second responsive to radiation propagating through shieldsection 37, etc. Hence, the detector array illustrated by FIG. 2comprises four channels, each having a different number of effectivedetector areas. The field of view of each of the four detector channelsis identical to the 60 angle between edges 33 and 34 and the arcuatearea of each detector channel capable of being irradiated by source 22is identical, being equal to 30 of arc.

To measure the position of source 22, each detector channel derives abinary signal indicative of the radiation level impinging thereon beingabove or below a threshold. Hence, in the situation illustrated by FIG.2, wherein source 22 is at an angle above horizontal line 30, thedetector channel associated with shield layer 36 receives a relativelysmall amount of radiation from source 22 because the shield area isinterposed between source 22 and the detector segment associated withshield segment 36. In contrast, the detector channel associated withshield 37 has no shield area interposed between the detector and thesource 22 and a considerably larger amount of radiation is detected.Similarly, the portions of detector 31 associated with shields 38 and 39receive insignificant and considerable radiation from source 22,respectively. Binary detectors connected to the detector channelsrespond to radiation from source 22 thereby derive binary signalsindicative of 0, 1, 0, and 1, respectively. In response to radiationsource 22 translating to a different position, wherein a differentcombination of shields is interposed between detector 31 and the source,a different binary signal is derived.

While only four shield sections are illustrated in FIG. 2, it isunderstood that in a system having a large resolution, more than fourshields are required. For example, in a practical system wherein theoutput of detector 31 is sampled once every 0.1 seconds and theelevation angle 1 of aircraft 21 relative to detector station 23 iscomputed to 0.6 minutes of arc, nine channels are employed to obtain acount rate of 31 per second for a slant range of approximately 300 feetbetween detector 23 and aircraft 21. The number of detector channels isrelated to resolution. Poisson's distribution equation, supra, can beused to determine which channel count rate is needed for adequatereliability.

To provide a more complete description and understanding of the detectorschematically illustrated in FIG. 2, reference is now made to FIGS. 3-7of the drawing. The illustrated detector array comprises stackedcylindrical detector segments 40-49, only six of which are illustratedto simplify the drawing. Detector segments 4049 are isolated from eachother electrically and have independent radiation responses to define 10separate detector channels. Independence in the radiation responses ofadjacent ones of detectors 40-49 is established by disc-shaped leadshields 51, one of which is positioned between each of the adjacentdetectors and extends to the periphery of shield body 32. I

Positioned between each of discs 51 and intercepting energy impinging ondetectors 4149 are cylindrical shield arrays 61-69, respectively. Eachof shield arrays 6l69 includes 2"' pie-shaped shield sections, where Nis the units digit associated with each shield array, e.g., shield arrayshield 61 includes 2' 1 segment, while shield array 64 consists of 2 8segments. Each shield in arrays 61-69 extends to the periphery ofsectored cylindrical background radiation shield 32 and has a radiusgreater than detectors 40-49, in contrast to the schematic showing inFIG. 2. The shields in each of the shield arrays subtend an equal angleand are separated from each other by equal-width pie-shaped gaps. Forexample, the eight shield segments comprising shield array 64 eachsubtend an angle h /16 and the gaps between them cover an equal angle,where 1 is the detector array field of view between edges 33 and 34. Noshield section is positioned in front of detector 40, utilized foracquisition purposes, as indicated supra.

The first channel, associated with shield array 61 and detector section41, is illustrated in cross section by FIG. 4. Shield array 61 isdivided into gap portion 70 and shield portion 71, which togethersubtend an angle of 60, between radially extending edges 33 and 34.Shield portion 71 occupies one-half of the area between edges 33 and 34,spanning the 30 of are from edge 33 to centerline 72 between edges 33and 34. Hence, shield 71 blocks radiation from source 22 when the sourceis within the detector array field of view above centerline 72,preventing radiation from reaching detectors 41 and a low level outputis derived from the arcuate surface of detector 41. In contrast, the gapportion between centerline 72 and edge 34 is transparent to penetratingradiation so that the presence of a radiation source in the are betweenline 72 and edge 34 irradiates detector 41 with relatively largeamountsof radiation.

The relationship between the angular position and the radiation countrate impinging on detector 41 is illustrated by FIG. 5, wherein countrate is plotted as the ordinate and angle as the abscissa of a Cartesiancoordinate system. In FIG. 5, the angular positions corresponding withedges 33 and 34 are indicated as D 1 and 1 0, respectively, while theangular position corresponding with centerline 72 is indicated as It isnoted from FIG. 5 that count rate as a function of angle has very sharpslopes, of virtually 90, at the angles 1 0 and and that between thesetwo angles the count rate remains constant. For angular positions lessthan Q, O and greater than the count rate does not drop to zero,however, because of the ever-present background radiation to which anyscintillation or semiconductor detector is exposed, as well as theinability of any shield to attenuate sufficiently radiation from asource and prevent all of the radiation from reaching detector 41. Thecount rate due to background and imperfect shielding from the source isgenerally on the order of four counts per second, a factor that issufficiently small relative to a count of 31 counts per second to begenerally ignored.

Thin, radially extending collimator plates 73 are placed in gap 70between centerline 72 and edge 34 in the shield structure 71 of FIG. 4to reduce the penumbra] shadow region, Ad shown in FIG. 7. Reduction inND is had with a sacrifice in count rate. Collimator plates 73 arepreferably made of a dense, but rigid, metal, such as tungsten to absorboff-axis radiation from source 22 that might otherwise impinge on theexposed portion of detector 41. Hence, if a radiation source werepositioned at point 74, FIG. 4, the shortest distance between point 74and detector 41 is through shield 71. However, there is a straight linepath from point 74 to detector 41 for energy following the path definedby line 74.1. Collimator plates 73 intercept and absorb the energyfollowing line 74.1, whereby the relatively vertical response indicatedby FIG. 5 is derived. Collimator plates 73 have no effect on radiationdirected radially toward detector 41, however, since they have nosubstantial cross section blocking the radiation path in such aninstance.

A perspective view of detector 42 and shield section 62, having twoshield segments 74 and 75, is illustrated in FIG. 6. Each of shieldsegments 74 and 75 defines an angular sector of approximately 15 and isseparated from the other by gap 76 of 15, while gap 77 separates shields74 and 32. Gaps 76 and 77 define two exposed areas on detector 42, incontrast with the single area defined on detector 41 by gap 70. Thereby,shields 74 and 75, with gaps 76 and 77, divide the arc ofdetector 42 inthe field of view of the second channel into four areas, two of whichcan be exposed directly to radiation from a source and two of whichcannot be directly irradiated by the source.

A plot of count rate derived from detector 42 as a function of angleacross the field of view of the second channel is indicated by FIG. 7.From FIG. 7, it is seen that for sources in the angular regionsrelatively large counts are derived, while low count rates are generatedwhen the source is outside of these boundaries.

Relatively steep slopes between the transition points on the plot of HG.7 are established by providing collimating plates 73 in gaps 76 and 77of the detector channel illustrated by FIG. 6, in a manner similar towhat is achieved with the detector channel illustrated by FIG. 4. Thenumber of collimator plates in each gap of the FIG. 4 configuration isless than the number of such plates provided in the single, relativelylarge gap of F IG. 4 because of the fewer off-axis radiation paths.

It is to be understood from the description of FIGS. 4-7 that theremaining seven detector channels include progressively larger number ofshields and gaps, with the number of gaps associated with each channelbeing commensurate with the number of the detector, i.e., detector 43receives radiation through four different gaps, detector 44 isresponsive to radiation directed through eight gaps, etc. The number ofplates per degree may be the same for every channel. As the number ofgaps increases, the opening in each decreases, whereby the number ofcollimator plates is reduced.

Reference is now made to FIG. 8 of the drawings, wherein a circuitdiagram of the apparatus utilized to derive an indication of the angularposition of radiation source 22, in conjunction with detectors 40-49 isillustrated. The circuit of FIG. 8 includes means for automaticallysetting the level of binary decision elements responsive to the outputof each detector as a function of range between detector station 23 andaircraft 21. To this end, the output of detector 40 feeds a rangecomputer 81. Range computer 81 provides a signal proportional to theslant distance between detector station 23 and aircraft 21.

The output of range computer 81 is applied to nine different binarylevel detectors 91-99, respectively responsive to the count ratesderived from detectors 41-49. To simplify the drawing, only detectors41, 42 and 49 for the channels having one, two and 256 shield gaps isshown. Similarly, only binary level detectors 91, 92 and 99,respectively responsive to the outputs of detectors 41, 42 and 49 arespecifically illustrated on FIG. 8.

Each of binary level detectors 91, 92, etc., includes a pair of outputleads for deriving binary zero or one signals, depending upon theamplitude of the signal applied to the detector by its respectiveradiation detector. Each of level detectors 91-99 counts or integratesthe signal from its corresponding radiation detector over apredetermined sampling time interval, equal to the interval betweenadjacent 50 millisecond pulses of 101-12 clock source 101. The output ofclock source 101 is applied in parallel to each of the detectors 91-99.If the number of counts impinging on a radiation detector 41-49 isgreater than an amplitude set in level detectors 91-99 during theinterval between adjacent pulses from clock source 101, the binary leveldetector derives a binary one output in response to a pulse from clocksource 101. In contrast, a binary zero output is derived from the'leveldetectors 91-99 if the count number is less than the detection levelthereof.

The binary ones and zeros generated by level detectors 91- -99 areapplied to readout means 102. Readout means 102 may take the form of avisual indicator, in which case it includes circuitry for converting thebinary coded signals from detectors 91-99 into a decimal signal forhuman presentation. in the alternative, readout means 102 may comprisethe input to a digital computer that responds to data from the otherdetectors of FIG. 1. The computer digitally calculates the elevation andground position of aircraft 21 in response to its several digital inputsignals.

To eliminate ambiguities which can occur if radiation from a source isblocked from detectors 41-49 while the source is in the field bounded bythe angles 1 and 1 1 as occurs in one position out of 2", the output ofdetector 40 is fed through binary level detector 103. Detector 103,indentical with each of detectors 91-99, derives a binary signal that iscombined with the outputs of detectors 91-99 in readout circhit-102. 1faradiation source isatanaggbetweenO g o'go there is a 1:256 probabilityof radiation impinging only on detector 40, which readout circuit 102provides.

Reference is riow made to FIG. 9 of the drawings wherein there isillustrated a preferred embodiment for the circuitry utilized in each ofthe binary level detectors 91-99 and 103. For purposes of explicitness,it is assumed that the detector illustrated by FIG. 9 is detector 91,which is responsive to the count output of detector 41. Detector 41 iscoupled to the input of RC integrator 106, having a very short chargingtime constant, on the order of 1 microsecond, but a relatively longdischarge time constant equal to the interval between adjacent pulsesfrom clock source 101. Such RC integrators are well known in the art,and generally comprise diodes feeding parallel resistance capacitancenetworks, hence need not be described in detail.

The output of RC integrator 106 is sampled once every 0.] seconds for a50-millisecond interval in response to the output of clock 101. Samplingis accomplished with normally closed gate 107 that is opened to enablethe voltage level at the output of integrator 106 to be passed to theinput of Schmidt trigger 108 only for the duration of the 50-millisecondclock pulses derived from source 10] once every tenth of a second.

The triggering level of Schmidt trigger 108 for the output of gate 107is variable, in response to the voltage level generated by rangecomputer 81. As the output signal derived from range computer 81increases to indicate greater ranges between aircraft 21 and detectorarray 23, the threshold level of Schmidt trigger 108 decreases. Schmidttrigger 108 responds to the variable amplitude pluses applied to it bygate 107 to derive constant amplitude, constant width pulses of approximately 75 milliseconds duration if the signal applied to it isabove the threshold value established by range computer 81. if thevoltage applied to Schmidt trigger 108 by RC integrator 106 is less thanthe threshold value established by range computer 81 during theSO-millisecond interval when gate 107 is opened, the trigger'outputvoltage remains at a constant, zero level.

The output of Schmidt trigger108 is sampled once each time that gate 107is opened to derive binary one and zero positive voltage indicatingsignals on leads 111 and 112. respectively. Sampling is accomplished byfeeding the Schmidt trigger output in parallel to one input of AND gate113 and the inhibit input of inhibit gate 114. Each of gates 113 and 114is strobed with a pulse from clock source 101 once every 0.1 seconds fora 50-millisecond interval, whereby there are derived positive and zerovoltages on lead 111 if Schmidt trigger 108 is activated in response tothe output of RC .integrator 106. in an opposite manner, zero andpositive volt ages are derived on leads 111 and 112 in response to theinput of Schmidt trigger 108 being less than the threshold valueestablished therein.

As an auxiliary or alternative feature, the threshold level of Schmidttrigger 108 can be manually adjusted by the voltage tapped from theslider of potentiometer 115, energized with a positive DC potential. Thevoltage at the slider of potentiometer 115 is linearly combined with theDC output voltage of range computer 81 to establish the threshold levelof Schmidt trigger 108.

The voltage tapped from potentiometer 115 can be set manually orautomatically as a function of height when the equipment is utilized formeasuring azimuth angle. Generally, it is necessary to vary thethreshold level of Schmidt trigger 108 as a function of elevation anglebecause, at higher elevation angles, smaller amounts of radiation passthrough the gaps in shield sections 61-69. Smaller amounts of radiationpass through shield sections 61-69 at higher elevations because thecollimators and shields have a tendency to shadow detector 31 fromsource 22. Hence, for elevations approaching while measuring azimuth,the threshold level of Schmidt trigger 108 is lowered, whereby a binaryone signal is derived on lead 111 in response to a lower count ratereceived by detector 42.

It is to be understood that the threshold value of Schmidt trigger 108can be controlled exclusively by manual means and that the position ofthe slider of potentiometer can be adjusted by hand as a function ofrange between detector array 23 and aircraft 21. In such a case, theslider of poten tiometer 115 is adjusted so that the threshold ofSchmidt trigger 108 is lowered for increasing ranges.

To provide an understanding ofthe manner in which the circuit of FIG. 9functions in conjunction with detector 42, having a pair of radiationtransparent gaps therein, reference is made to FIGSv 10 and 11. FIGS. 10and 11 respectively are plots of radiation level as a function of anglefor short and long ranges between aircraft 21 and detector array 23. Inaddition, FIGS. 10 and 11 indicate how: threshold value changes as afunction of range; stray radiation from source 22 propagating throughthe shield structure to detector 42 varies as a function ofdistance; andbackground radiation remains constant.

In FIG. 10, pulses 121 and 122, having a relatively large amplitudecompared to base line 123, are derived for the angular spans It isassumed that aircraft 21 carrying source is close to the detector,whereby the average count impinging on detector 42 for the angle definedby levels 121 and 122 is 40 per second. In response to the relativelyhigh count rate detected with aircraft 21 so close to detector array 23,range computer 81 derives a relatively small voltage that sets thethreshold of trigger 108 to a relatively large value of 12 counts persecond, as defined by line 124. The background count rate is assumed tobe 4 per second while 3 counts per second are assumed to propagatethrough the shield structure to detector 42 with the relatively smallseparation between aircraft 21 and detector 23.

FIG. 11 indicates the manner in which threshold line 124 decreases to 8counts per second when aircraft 21 becomes removed sufficiently fromdetector 23 to cause the detector 42 count rate to drop to 20 per secondin the angular region defined by the gaps Similarly, the transmitivitythrough the shield structure to detector 42 drops from 7 toapproximately 5 /zcounts per second with increased range, whilebackground radiation level remains constant at 4 counts per second.Because the threshold level 124 has dropped in FIG. 11, the probabilityof deriving a binary one output from AND gate 113 remains approximatelythe same as under the conditions existing with the waveform of FIG. 10.A decrease in probability does not occur even if the count rate shoulddrop for a relatively short time interval because of the variablethreshold. The lower threshold, however, can cause a greater probabilityof binary ones on output lead 111 of the network of FIG. 9 if backgroundradiation should increase. It is generally understood, however, thatbackground radiation is a long term phenomena, is not subject toappreciable variations and can be predicted on an a priori basis. Hence,the level of detector 108 can be preset to indicate background withgreat reliability.

Reference is now made to FIGS. l214 wherein there is illustrated still afurther embodiment of the present invention. The embodiment of FIGS.12-14 is particularly adapted for measuring elevation angleindependently of azimuth, that is, elevation angle is measured with thesame response regardless of the azimuth angle of aircraft 21.

In the embodiment of FIGS. 1214, l0 separate detector channels 130-139are provided in stacked vertical relation ship. As in the embodimentdiscussed supra, each of the detector channels 130139 has an identicalfield of view from an angle 1 0 to I 1 and includes a different numberof gaps for enabling penetrating radiation to be transmitted to ascintillation or semiconductor detector.

A typical detector in the array of FIG. 14 is disclosed in perspectiveand cross-sectional views by FIGS. 12 and 13, respectively. The detectorillustrated by FIGS. 12 and 13 includes a pair of gaps 141 and 142 forenabling radiation to reach detector 143. The shield structure isdefined by rotating the cross section illustrated by FIG. 13 about thevertical axis 144 that extends through the center of detector 143.Thereby, a figure of revolution comprising two main annular shieldsections 145 and 146 and a disclike shield section 147 is provided. Gaps141 and 142 are formed'between the two main shield sections 145 and 146and the disclike section 147. Disposed in the gaps are collimator plates148 which are constructed and function similarly to collimator plates73, FIG. 4. Because the detector shield of FIG. 13 has a 360 field ofview relative to the azimuth angle, radiation impinges equally on allsegments of detector 143 regardless of the azimuth position of aircraft21 relative to detector array 23.

It is to be understood that the remaining detectors in the stacked arrayillustrated by FIG. 14 are constructed similarly to the shield for thesecond channel illustrated by FIGS. 12 and 13. Of course, each oftheother channels includes a shield having a different number of gaps forenabling radiation to reach the detector located therein from the twogap arrangement illustrated by FIGS. 12 and 13.

Reference is now made to FIGS. 15 and 16 of the drawings wherein thereis illustrated another embodiment of the present invention whereinplanar, rather than circular, shields and detectors are employed. In theembodiment of FIGS. 15 and 16, as well as the remaining plural channelembodiments to be described, only three separate position-indicatingchannels are specifically illustrated, for the purposes of simplicity.It is to be understood that the number of channels should be increased,e.g., to nine, and that the resolution of the system increases as anexponential function ofthe number of channels.

The detector of FIGS. 15 and 16 comprises four elongated, planardetector arrays 200-203 responsive to energy from a random source, suchas gamma-rays or X-rays. Each ofdetector arrays 200-203 has the samelength and width, and mutually parallel longitudinal axes, so that theyare in side-byside adjacent relationship. Positioned to interceptpenetrating radiation that would otherwise impinge on detectors 201- 203are shielding arrays 204206, respectively.

Each of the shields in shield arrays 204-206 is located across theentire width of its corresponding detector and is located on thenormally exposed detector surface. Shielding array 204 includes a singleshield that extends from the center of detector 201 to its uppersurface. Shielding array 205 includes a pair of shields, each having alength equal to onefourth of the length of detector 202 and positionedso that equal A-length segments of the detector 202 can be illuminatedby radiation. Detector array 206 includes four spaced shields, eachhaving alength one-eighth ofthe length of detec-' tor 203 to define fourgaps along the detector responsive to penetrating radiation.

Positioned between the detector arrays and source 208 of penetratingradiation carried on a vehicle, the angular position of which is beingmonitored, is slit plate 207. Slit plate 207, mounted in a planeparallel to the detector array, is a shield to penetrating radiationfrom source 208 and includes a centrally located elongated slit 209,extending substantially across its width.

The vertical position of horizontally extending slit 209 is determinedby the 'use to which the angular detector of FIGS. 15 and 16 is made.For measuring an object that is always located vertically above thedetector array, such as a groundbased aircraft angular positiondetector, the longitudinal axis of slit 209 is positioned substantiallyabove the centerline through detectors 201-203, as defined by the loweredge of shield 204. If the angular detector, however, is employed in asystem wherein the radiation source is in the same horizontal andvertical plane as the detector array, slit 209 lies on the intersectionbetween the lower surface of shield 204 and the exposed portion ofdetector 201.

In any event, the field of view to which the detector array of FIGS. and16 responds is defined by the lines 211 and 212, extending through slit209 from the upper and lower edges of the detector array, respectively.To enlarge the angular field of view, slit plate 207 is moved closer tothe detector array and the detector array is enlarged along thelongitudinal axes of each of detectors 200-203. Enlarging the detectorarray by translating shield 207 to closer proximity with the detectorarray, however, has the disadvantage of reducing the resolution of thesystem. Reduction in resolution occurs because off-axis radiation fromsource 208 is not collimated by slit 209 when shield 207 is in closeproximity to the array. Because radiation from source 208 is notcollimated when it impinges on the detector array for a relatively closespacing between the array and shield 207, a relatively wide radiationbeam impinges on the channels associated with detectors 200203, alongthe detector longitudinal axes. Of course, if a relatively wide beamimpinges on the detector array, the possibility of ambiguous readoutoccurs because the beam may impinge on two exposed detector portions ina single channel, particularly in the highest resolution channelswherein spacing between exposed detector areas is quite small.

In the configuration of FIGS. 15 and 16, detector 200, having no shieldinterposed between it and shield 207, is utilized for obtaininginformation indicative of the presence or absence of a radiation source,in the same manner as detector 40 is utilized in the previouslydescribed embodiments. To prevent radiation from source 208 impinging onthe detector array when the source is out of the field of view definedby lines 211 and 212, shield plates 213 and 214 extend from the upperand lower edges of shield 207 to the upper and lower edges,respectively, of the detector array. Similar shields, not shown, extendfrom the left and right edges of slit plate 207 to the left and rightedges of the detector array.

The outputs of detectors 200203 are applied to a circuit identical tothat illustrated in FIG. 8 to derive a digital indication of theposition of radiation source 208.

Reference is now made to FIGS. 17 and 18 of the drawings, wherein thereis illustrated another embodiment of the invention employing a planardetector array. In the detector of FIGS. 17 and 18, only three detectorchannels 221, 222 and 223 are provided. Each of channels 221-223includes two groups of detectors, with channel 221 including twoadjacent detectors 224, 225 having common longitudinal axes. In thefirst group of detector channel 222 are included two detectors 226 and227, separated by the second detector group, comprising detectors 228and 229. In detector channel 223, the first detector group includesdetectors 231-234 separated from each other by the detectors of secondgroup, including detectors 235-238. The detectors in each of groups 221--223 are longitudinally aligned, with the detectors of the first groupalways being separated by the detectors of the second group, and anequal number of detectors in each group being provided in each channel.The several channels are in abutting relationship, in a manner similarto the embodiment of FIG. 15.

The detector array of FIG. 17 has the advantage over the system of FIG.15 of increased accuracy because reliance is not made upon the absenceof a signal on one of the detectors for information. Instead, each ofchannels 221223 includes detector groups that cover the entire length ofthe channel. In addition, since a definite indication of energyimpinging on each channel is derived, there is no need for a detectorchannel, such as channel 200, in the system of FIGS. 15 and 16.

The system of FIGS. 17 and 18 differs in a more important manner fromthe configuration of FIGS. 15 and 16, however, by virtue of shadow maskor shield 241 that is positioned between the detector arrayandpenetrating radiation source 242. Shield 241 is positioned in a planeparallel to the detector array to intercept a portion of the energy fromsource 242. Shield 241 prevents substantially all of theradiation-responsive array below a predetermined line, determined by theposition of source 242, from being irradiated. The remainder of theradiation-responsive array, above the horizontally extending shadow linedefined by the upper edge of shield 24] is illuminated by source 242. Bymeasuring the demarcation line between the illuminated andnonilluminated portions of the detector array, the angular position ofsource 242 is determined.

The shadow edge system of FIGS. 17 and 18 has the advantage over theslit plate array of FIGS. 15 and 16 in that the former system does notrely upon collimating the radiation source as it impinges upon thedetector array. With the detector of FIGS. 17 and 18, energy from source242 need not impinge on the detector array in parallel lines to define arelatively narrow irradiated strip across the width of the detectorarray. Instead, a relatively large area of the detector array isilluminated.

To prevent possible spurious results-from being derived with the systemof FIGS. 17 and 18, shield 243 extends from the lower edge of the arrayhorizontally to shield 241, whereby radiation from source :242 cannot becoupled to the detector array ifthe source is at an angle below theupper edge of shield 241.

One preferred embodiment for detecting the demarcation line between theirradiated and nonirradiated detector portions of the system of FIGS. 17and 18 is illustrated by the circuit diagram of FIG. 19. In the circuitof FIG. 19, the detectors in channels 221223 are arranged so that thedetectors of each channel are similarly grouped. The output of each ofthe separate detectors is fed to a separate threshold detector 244, ofthe type illustrated by FIG. 9, except that no range com puter isemployed. Thereby, each of threshold detectors 244 derives a bilevelvoltage output indicating whether any radiation from source 242 impingeson the radiation detector which feeds radiation counts to it.

The outputs of detectors 244 are fed to three differential DC amplifiers245247. The voltages derived from amplifiers 245-247 indicate whether aneven or odd number of radiation detectors in each of groups 221-223 isilluminated in response to energy from source 242. Each of the thresholddetectors feeding input signals to amplifiers 245247 is connected withan output resistance, each of which is equal in value so that equalamplitude voltages are applied by each detector to the amplifier inputterminals.

To achieve the amplifier bilevel output signals, threshold detectors244, driven by radiation detectors 224 and 225, in channel 221, feed theplus and minus input terminals, respectively, of differential amplifier245. Threshold detectors 244, responsive to the radiation detectors 226and 227 in the first group of detectors in count 222, feed the plusinput terminal of differential amplifier 246 while the minus inputterminal of differential amplifier 246 is responsive to the sum of thebilevel voltages derived from the remaining threshold detectors in thesecond group. Similarly; the plus input terminal of differentialamplifier 247 is responsive to the threshold detector outputs that arefed by detectors 231-234 of the first group of detectors in the thirdchannel 223 while the minus input terminal of differential amplifier 247is responsive to the bilevel signals derived in response to detectors235238 in the second group of detectors 223. In response to the positiveand negative voltages applied to their inputs, amplifiers 245- -247derive bilevel output voltages, having values of either zero or a fixedpositive amplitude.

To provide an understanding as to the functioning of FIGS. 17-19, it isassumed that the demarcation line between the irradiated andnonirradiated portions of the detector array of FIG. 17 is defined byline 248, passing through detectors 224, 228 and 232. In response tosuch irradiation, each of detectors 224, 226, 228, 231, 232 and 235derives a signal having a sufficient count rate to cause the thresholddetectors fed thereby to generate a positive bilevel output. Theremaining detectors in the array, viz., detectors 225, 227, 229, 233,234, 236, 237 and 238, are insufficiently irradiated to cause thethreshold detectors driven thereby to derive binary one signals. Inconsequence, zero voltage signals are generated by each of thesethreshold detectors.

In response to the high and low count rates derived from detectors 224and 225, positive and zero voltage amplitudes are applied to thepositive and negative input terminals of differential amplifier 245,whereby the amplifier is driven to generate a positive constant leveloutput voltage. Amplifier 246, however, responds to its inputs to derivea zero voltage output because equal positive voltages are applied to itspositive and negative input terminals by the threshold detectors drivenby radiation detectors 226 and 228. The threshold detectors driven byradiation detectors 227 and 229 generate zero output voltages that haveno effect on the input voltages applied to amplifier 246.

Radiation detectors 231 and 232 derive outputs whereby a pair ofpositive voltages are applied to the positive input terminal ofdifferential amplifier 247 while a single positive voltage is applied tothe negative terminal of amplifier 247 in response to the signal derivedby radiation detector 235. Since the positive input terminal ofamplifier 247 has a greater voltage applied thereto than the negativeinput terminal of the amplifier, the amplifier output is a positivevoltage.

The bilevel outputs of amplifiers 245247 are applied to readout circuit248, which converts the parallel binary signal to a human readabledecimal indication.

Reference is now made to FIG. of the drawings, wherein there isillustrated a further embodiment of the present invention. In FIG. 20,shadow mask or shield 241 is utilized in conjunction with a singledetector channel having a relatively high resolution, for example,detector channel 223 of FIG. 17. The field of view covered by detectorchannel 223, including detectors 231-238, is between the lines 251 and252 and covers an angle from to +4 relative to the horizontal. Theoutput of each of detectors 23l238 is applied to the input of thresholddetectors 261268, respectively, each of which derives a bilevel input asdescribed supra in conjunction with FIGS. 9 and 19.

The bilevel outputs derived by threshold detectors 261- -264 are appliedin parallel through summing resistors to the positive input terminal ofdifferential amplifier 269 while the outputs of threshold detectors265268, for the second group of detectors in channel 223, are applied tothe negative input terminal of differential amplifier 269. Differentialamplifier 269 responds to the difference of the voltages applied to itsinput terminals to derive a bilevel output voltage, in a manner similarto amplifier 247, FIG. 19.

The output voltage of amplifier 269 is coupled to the count advanceinput of counter 271, which serves as a memory to indicate the positionof radiation source 272. Counter 271 is advanced in response to eachtransition in the output voltage of amplifier 269.

Counter 271 is selectively set to a count commensurate with 23?, inaccordance with the position of radiation source 272 prior to theradiation source coming into the detector field of view between lines251 and 252. If radiation source 272 is initially outside the definingfield of view between lines 251 and 252, at an angle above the horizongreater than 1 counter 271 is set to the l state. With counter 271 setto the +4 state, it is activated so that the count stored therein isdecremented in response to each transition in the output voltageamplifier 269. In an opposite manner, if radiation source 272 is outsideof the field of view because it is at a vertical angle greater than 1counter 271 is set to a count of 1 and is activated so that its count isincremented, rather than decremented, in response to each transitionfrom the output of amplifier 269. A

To these ends, the voltages applied to the positive and negative inputterminals of differential amplifier 269 are summed together in amplifier273. The output voltage of amplifier 273 is fed to trigger circuit 274.The triggering level of circuit 274 is adjusted whereby the triggerderives a positive voltage in response to the output of amplifier 273being greater than a predetermined level that indicates each ofradiation detectors 231-238 is illuminated by radiation from source 272.The output of trigger circuit 274 is applied to input 275 of counter271, which input, when activated with a positive voltage, sets thecounter to a state corresponding with In response to the positive outputvoltage of trigger 274, input 275 energizes counter 271 so it isdecremented in response to each transition in the output voltage ofamplifier 269. Counter 271 is set to its second initial condition,whereby a value of I is preloaded therein and it is energized so itscount is incremented in response to each transition in the outputvoltage of amplifier 269, by NAND gate 276 feeding a positive voltage tothe counter input terminal 277. NAND gate 276 is fed by the outputs ofthreshold detector 261 and amplifier 269, to derive a positive outputvoltage only when its two input signals are of zero voltage value,

The count stored incounter 271 is applied to readout means 278, whichincludes the necessary conversion to enable a decimal display to bederived.

To provide a better and more complete understanding as to the manner inwhich the system of FIG. 20 functions, a number of examples will begiven, first presuming that radiation source 272 moves downwardly from aposition above the angle 1 With radiation source 272 initially aboveline 252, i.e., at an angle greater than 1 each of detectors 231238 isirradiated. In response to irradiation from each of detectors 231238,each of threshold detectors 261-268 generates a positive output voltageof predetermined magnitude. The positive output voltages generated byeach of detectors 261- 268 are applied to the input terminals of summingamplifier 273, which drives trigger circuit 274 to a positive outputvoltage.

The positive output voltage of trigger circuit 274 is coupled to inputterminal 275 of counter 271, setting the counter to a count commensuratewith the angle D The positive voltage applied to terminal275 also causescounter 27! to be set so that it is decremented in response to eachsubsequent transition in the bilevel output of amplifier 269. Thepositive output voltage of trigger 274 maintains counter 271 activatedas stated until radiation source 272 passes to an angle where shield 241prevents irradiation by detector 238 from source 274.

In response the detector 238 being shadowed from source 272 by shield241 when source 272 moves to the sector between lines 281 and, 282,threshold detector 268 is switched from a positive to a zero outputvoltage. In response to the output voltage of threshold detector 268dropping to a zero voltage, the output voltage of amplifier 273 becomesless than the threshold value of trigger circuit 274, whereby thetrigger circuit output voltage goes to zero, enabling the count incounter 271 to be changed in response to transitions in the outputvoltage of amplifier 269. Counter 27] includes memory means so that itremains activated in the decrement mode, whereby each transition in theamplifier 269 output causes the count stored in the counter to bereduced by one.

Simultaneously with the output voltage of trigger 274 being reduced tothe zero voltage level, a difference in the voltages applied to thepositive and negative input terminals of amplifier 269 occurs. Thedifference in the voltages applied to the input terminals of amplifiers269 occurs in response to each of threshold detectors 261-264 deriving apositive voltage, while only threshold detectors 265-267 feed positivevoltages to the negative input terminal of the amplifier.

In responseto the difference voltage at the input terminals of amplifier269, the amplifier derives a finite positive amplitude output signal.The output signal derived from amplifier 269 is delayed slightly withrespect to the transition in the output of trigger circuit 274, wherebycounter 271 is positively driven by the positive-going transition in theoutput voltage of amplifier 269 after the trigger output level hasdropped to zero. In response to the positive-going transition in theoutput voltage of amplifier 269, the state of counter 271 is decrementedby a count of one and an indication is derived from readout means 278that radiation source 272 is in the field defined by lines 281 and 282.

As radiation source 272 continues downwardly it crosses line 282,whereby radiation detector 234 is no longer irradiated. In response tocessation of irradiation of detector 234 by source 272, the outputvoltage of threshold detector 264 drops to the zero voltage level and anegative going transition occurs in the output voltage of amplifier 269.The negative going transition in the output voltage of amplifier 269activates counter 271 so that the count stored therein is decrementedagain, whereby readout 278 provides an indication that radiation source272 is in the field of view between lines 282 and 283. In the mannerdescribed, it is believed obvious as to the manner in which counter 271responds to radiation from source 272 and reeds readout means 278 toprovide an indication of the angular position of the radiation source.

Once radiation source 272 descends vertically to an angle less than Ii.e., is below line 251, no radiation impinging on detectors 231-238. Inresponse to zero radiation impinging on each of detectors 231238, eachof threshold detectors 261-268 derives a zero voltage output level. Thezero voltage output levels of threshold detectors 261268 feed thepositive and negative input terminals of amplifier 269, whereby theamplifier derives a zero output voltage. The zero voltage output levelof amplifier 269 is fed to NAND gate 276 that is also responsive to thezero voltage output of threshold detector 261. Since both inputs to NANDgate 276 are zero voltages, the NAND gate generates a positive outputvoltage, setting counter 271 to a I state and energizing the counter sothat it is incremented in response to subsequent transitions in theoutput voltage of amplifier 269.

If radiation source 272 should now return into the field of view ofdetectors 231-238 by crossing line 251 in the upward direction,radiation detector 231 is illuminated. In response to irradiation ofdetector 231 by source 272, threshold detector 261 derives a positiveoutput voltage, whereby a positive transition is derived at the outputterminal of amplifier 269 and the input voltage of counter 271, atterminal 277, drops to a zero level. In response to the positivetransition at the output terminal of amplifier 269, counter 271 isincremented by a count of one, to derive an indication that source 272is between the angle defined by lines 251 and 284. In the mannerdescribed, counter 271 responds to succeeding illumination of thedetectors in channel 223, whereby readout means 278 provides a decimalindication of the angular position of source 272 as it traversesupwardly.

Reference is now made to FIG. 21 of the drawings, wherein there isillustrated a further embodiment of the invention. The embodiment ofFIG. 21 is very similar to that of FIG. 17 in that it includes shadowmask 241 and does not require a detector channel for acquisitionpurposes solely. The system of FIG. 21 differs from the detector arrayof FIG. 17, however, since only one detector per channel is required.

In particular, each of channels 221, 222 and 223 includes a singledetector 291, 292 and 293, respectively. Positioned between the detectorarray and shadow mask 241 is a separate shield array 294-296 for each ofdetectors 291-293, respectively. Shield arrays 294-296 are arrangedprecisely in the same manner as shield arrays 204-206, FIG. 15, todefine equal radiation fields of view for each of the detector channelsand a different number of effective detector areas in each channel.

The system of FIG. 21 does not require an acquisition detector, such asdetector 200, FIG. 15, because the upper surface of shadow mask 241 ispositioned so that radiation must fall on the upper surfaces of each ofdetectors 291, 292 and 293. Hence, if the radiation source is anywherein the field of view, at least one of the detectors 29l293 isilluminated. It is thus seen that the system of FIG. 21 incorporates theadvantages of the system of FIG. 18 relating to lack of collimatingrequirements, but avoids the complexities of the multidetector arraythereof.

Reference is now made to the circuit diagram of FIG. 22, particularlydesigned to provide a decimal readout for the positional data derivedfrom the detector array of FIG. 21. In the circuit of FIG. 22, detectors291293 feed threshold detectors 301-303, respectively. Thresholddetector 301 is substantially the same as the threshold detectorillustrated by FIG. 9, except the range computer is excluded.

Threshold detectors 302 and 303 in the circuit of FIG. 22, however, arematerially different from the threshold detector 301. Detectors 302 and303 derive a bilevel transition in response to each step function changein the radiation level impinging on detectors 292 and 293, respectively.Since each of threshold detectors 302 and 303 is identical, adescription of detector 302 suffices.

Threshold detector 302 includes integrators 304 and 305, driven inparallel by the output of radiation detector 292. Integrator 304includes a relatively long time constant charging circuit and a shorttime constant discharge circuit while the converse is true of integrator305, i.e., integrator 305 has a short time constant charging circuit anda long time constant discharge circuit. Thereby, in response to a stepfunction increase in the count rate derived by detector 292, the outputvoltage of integrator 304 does not change suddenly, but the outputvoltage of integrator 305 increases at a relatively rapid rate. Incontrast, a sudden decrease in the count rate derived from radiationdetector 292 results in a sudden decrease in the output voltage ofintegrator 304 and a slow transition in the voltage generated byintegrator 305.

The output voltages of integrators 304 and 305 are applied todifferentiators 306 and 307, respectively. Differentiators 306 and 307are connected so that sudden transitions in the output voltages ofintegrators 304 and 305 in response to step function variations in theradiations level impinging on detector 292 cause relatively largevoltage outputs to be generated by the differentiators.

The outputs of differentiators 306 and 307 are linearly combined insumming network 308, the output of which feeds flipflop 309. Flip-flop309 responds to each pulse fed thereto by summing network 308, wherebythe flip-flop state is changed in response to each output pulse fromsumming network 308. It is thereby seen that the output voltage offlip-flop 309 is a bilevel wave train having transitions correspondingwith transitions in the radiation level impinging on radiation detector292.

The bilevel outputs of detectors 301303 are applied as parallel binaryinputs to binary to decimal converter and readout means 311. Thereby, avisual decimal indication is provided of the angular position of aradiation source illuminating or irradiating the detector array of FIG.21.

While I have described and illustrated several specific embodiments ofmy invention, it will be clear that variations of details ofconstruction which are specifically illustrated and described may bemade without departing from the true spirit and scope of the inventionas defined in the appended claims. For example, the spacing of adjacentgaps in the various shield members need not be uniform but can benonlinear, for example, to derive direct indications of the sine of anangle, or if greater resolution is required in certain angular regionsthan in others. It is also to be understood that digital, rather thananalogue techniques can be utilized for deriving signals indicative ofthe count rate picked up by each detector. In accordance with a furthermodification, all of the detectors in the embodiment of FIGS. 3-8 neednot be in stacked relationship, but they can be divided in severalstacks, each having substantially the same field of view.

The count rates are illustrated by way of example only and values otherthan that described may be found depending on the specific sourcestrengths, detector construction and electronic data-handling equipmentemployed.

Iclaim:

1. A system for indicating the angular position of a source ofpenetrating radiation, comprising a plurality of detector channels forradiation from said source, each of said channels covering substantiallythe same field of view relative to said source and having differingnumbers of detector areas susceptible to exposure to radiation from saidsource, means for selectively blocking the passage of said radiation tothe detector areas of said channels as a function of the source angularposition, and means for deriving a signal from each of said channelsindicative of the amount of said radiation received by the exposeddetector areas of each respective channel, said means for deriving,including means for generating a bilevel signal in response to the totalnumber of counts of said radiation impinging on each of said channelswithin a predetermined time interval, the number of counts per saidpredetermined time interval to derive an indication of a source being inthe field of view for each channel being related to the number ofchannels so that the amount of information derived from all of thechannels increases exponentially as the number of channels increases.

2. The system of claim 1 wherein Q of said channels are provided, theNth one of said channels including MX2-" exposed areas, where M is apositive integer equal to or greater than one, Q is a positive integergreater than one, and N is selectively all integers from one to Q,inclusive.

3. The system of claim 1 wherein adjacent ones of said detector areas ineach channel are separated from each other by areas unresponsive toradiation from said source.

4. The system of claim 3 wherein said areas unresponsive to saidradiation are detector areas covered by shields for said radiation, saidshields comprising said blocking means, said exposed areas and thesurfaces of said shields being arcuate in cross section, the arcuatesurfaces of said shields in each channel having greater radii than theexposed detector areas.

5. The system of claim 4 further including radially extending collimatorplates between adjacent shields in one of said channels.

6. The system of claim 5 wherein said exposed detector areas and arcuateshields are cylindrical.

7. The system of claim 5 wherein said detector areas and arcuate shieldsare defined as figures of revolution.

8. The system of claim 1 further including an additional detectorchannel having the same field of view as each of said plurality ofdetector channels, said additional detector channel being always exposedto radiation from said source anywhere in said field of view.

9. The system of claim 1 wherein said blocking means comprises shieldmeans for said radiation positioned between said source and detectorareas, said shield means including slit means for passing relativelynarrow width beams of said radiation to each of said channels.

10. The system of claim 1 wherein said blocking means includes shieldmeans for said radiation positioned between said source and detectorareas of said channels, said shield means having an edge positioned todefine a demarcation line for irradiated and nonirradiated detectorareas of said channels determined by the position of said source, all ofthe detector areas of said channels on one side of said line beingirradiated by said source and all of the detector areas of said channelson the other side of said line being shadowed from said source by saidshield means.

11. The system of claim wherein adjacent ones of said detector areas ineach channel are separated from each other by areas unresponsive toradiation from said source, said detector areas on said one side havingan exposed detector area in each channel always exposed to radiationfrom said source when the source is in said field of view.

12. The system of claim 1 wherein the detector areas of each channel aresubstantially coplanar and aligned, each of said channels havinggenerally parallel longitudinal axes.

13. The system of claim 1 wherein each of said channels includes twogroups of detector areas, each detector of each group being adjacent toa detector of the other group, each detector of each group beingseparated from another detector of that group by a detector of the othergroup.

14. The system of claim 13 wherein said means for deriving for eachchannel includes means for comparing the amount of said radiationimpinging on the first and second detector groups of each channel.

ing includes means for deriving only a bilevel signal in response to theamount of said ra ration impinging on each of the exposed areas, andmeans for linearly combining each of the bilevel signals derived fromthe exposed areas of each channel.

16. The system of claim 15 wherein said means for deriving includesmeans for deriving first and second bilevel signals from each of theexposed areas of said first and second groups. said combining meansincluding means for linearly combining said first signal and means forlinearly combining said second signals. 47

17. A detector for indicating the angular position of a source ofpenetrating radiation comprising a plurality of detector channels forradiation from said source, each of said channels covering substantiallythe same field of view relative to said source and having differingnumbers of detector areas susceptible to exposure to radiation from saidsource, and means for selectively blocking the passage of said radiationto the detector areas of said channels as a function of the sourceangular position wherein said blocking means includes shield means forsaid radiation positioned between said source and detector areas, saidshield means having an edge positioned to define a demarcation line forirradiated and nonirradiated exposed detector areas determined by theposition of said source, all of the exposed areas on one side of saidline being irradiated by said source and all of the exposed areas on theother side of said line being shielded from said source be said shieldmeans.

18. The system of claim 17 wherein adjacent ones of said detector areasin each channel are separated from each other by areas unresponsive toradiation from said source, said detector areas on said one side havingan exposed detector area in each channel always exposed to radiationfrom said source when the source is in said field of view.

19. The system of claim 17 wherein each of said channels includes twogroups of detector areas, each detector of each group being adjacent toa detector of the other group, each detector of each group beingseparated from another detector of that group by a detector of the othergroup.

20. A detector for indicating the angular position of a source ofpenetrating radiation comprising a detector channel for said radiationhaving a predetermined field of view, a multiplicity of different,separate detector areas within said channel each of said areas beingsusceptible to exposure to said radiation to derive an output signal inresponse to said radiation impinging thereon, different ones of saidareas being irradiated by said source for differing positions of saidsource relative to said channel, and shield means for said radiationpositioned between said source and detector areas, said shield meanshaving an edge positioned to define a demarcation line for irradiatedand nonirradiated detector areas of said channel determined by theposition of said source, all of the detector areas on one side of saidline being irradiated by said source and all of the detector areas onthe other side of said line being shielded from said source by saidshield means.

21. The system of claim 20 wherein said multiplicity of detector areasare separated from each other by a plurality of second detector areas,means responsive to the amount of radiation impinging on each of saidplurality of areas for de riving first and second bilevel signalsindicative of the total radiation impinging on each area being greaterthan a set level within a predetermined time interval, and means forcomparing the sum of the first-named bilevel signal with the sum of thesecond named bilevel signal.

22. The system of claim 20 wherein adjacent ones of said detector areasin said channel are separated from each other by areas unresponsive toradiation from said source, said detector areas on said one side havinga detector area in said channel always exposed to radiation from saidsource when the source is in said field of view.

1. A system for indicating the angular position of a source ofpenetrating radiation, comprising a plurality of detector channels forradiation from said source, each of said channels covering substantiallythe same field of view relative to said source and having differingnumbers of detector areas susceptible to exposure to radiation from saidsource, means for selectively blocking the passage of said radiation tothe detector areas of said channels as a function of the source angularposition, and means for deriving a signal from each of said channelsindicative of the amount of said radiation received by the exposeddetector areas of each respective channel, said means for deriving,including means for generating a bilevel signal in response to the totalnumber of counts of said radiation impinging on each of said channelswithin a predetermined time interval, the number of counts per saidpredetermined time interval to derive an indication of a source being inthe field of view for each channel being related to the number ofchannels so that the amount of information derived from all of thechannels increases exponentially as the number of channels increases. 2.The system of claim 1 wherein Q of said channels are provided, the Nthone of said channels including M X 2N-1 exposed areas, where M is apositive integer equal to or greater than one, Q is a positive integergreater than one, and N is selectively all integers from one to Q,inclusive.
 3. The system of claim 1 wherein adjacent ones of saiddetector areas in each channel are separated from each other by areasunresponsive to radiation from said source.
 4. The system of claim 3wherein said areas unresponsive to said radiation are detector areascovered by shields for said radiation, said shields comprising saidblocking means, said exposed areas and the surfaces of said shieldsbeing arcuate in cross section, the arcuate surfaces of said shields ineach channel having greater radii than the exposed detector areas. 5.The system of claim 4 further including radially extending collimatorplates between adjacent shields in one of said channels.
 6. The systemof claim 5 wherein said exposed detector areas and arcuate shields arecylindrical.
 7. The system of claim 5 wherein said detector areas andarcuate shields are defined as figures of revolution.
 8. The system ofclaim 1 further including an additional detector channel having the samefield of view as each of said plurality of detector channels, saidadditional detector channel being always exposed to radiation from saidsource anywhere in said field of view.
 9. The system of claim 1 whereinsaid blocking means comprises shield means for said radiation positionedbetween said source and detector areas, said shield means including slitmeans for passing relatively narrow width beams of said radiation toeach of said channels.
 10. The system of claim 1 wherein said blockingmeans includes shield means for said radiation positioned between saidsource and detector areas of said channels, said shield means having anedge positioned to define a demarcation line for irradiated andnonirradiated detector areas of said channels determined by the positionof said source, alL of the detector areas of said channels on one sideof said line being irradiated by said source and all of the detectorareas of said channels on the other side of said line being shadowedfrom said source by said shield means.
 11. The system of claim 10wherein adjacent ones of said detector areas in each channel areseparated from each other by areas unresponsive to radiation from saidsource, said detector areas on said one side having an exposed detectorarea in each channel always exposed to radiation from said source whenthe source is in said field of view.
 12. The system of claim 1 whereinthe detector areas of each channel are substantially coplanar andaligned, each of said channels having generally parallel longitudinalaxes.
 13. The system of claim 1 wherein each of said channels includestwo groups of detector areas, each detector of each group being adjacentto a detector of the other group, each detector of each group beingseparated from another detector of that group by a detector of the othergroup.
 14. The system of claim 13 wherein said means for deriving foreach channel includes means for comparing the amount of said radiationimpinging on the first and second detector groups of each channel. 15.The system of claim 14 wherein said means for comparing includes meansfor deriving only a bilevel signal in response to the amount of saidradiation impinging on each of the exposed areas, and means for linearlycombining each of the bilevel signals derived from the exposed areas ofeach channel.
 16. The system of claim 15 wherein said means for derivingincludes means for deriving first and second bilevel signals from eachof the exposed areas of said first and second groups, said combiningmeans including means for linearly combining said first signal and meansfor linearly combining said second signals. 47
 17. A detector forindicating the angular position of a source of penetrating radiationcomprising a plurality of detector channels for radiation from saidsource, each of said channels covering substantially the same field ofview relative to said source and having differing numbers of detectorareas susceptible to exposure to radiation from said source, and meansfor selectively blocking the passage of said radiation to the detectorareas of said channels as a function of the source angular positionwherein said blocking means includes shield means for said radiationpositioned between said source and detector areas, said shield meanshaving an edge positioned to define a demarcation line for irradiatedand nonirradiated exposed detector areas determined by the position ofsaid source, all of the exposed areas on one side of said line beingirradiated by said source and all of the exposed areas on the other sideof said line being shielded from said source be said shield means. 18.The system of claim 17 wherein adjacent ones of said detector areas ineach channel are separated from each other by areas unresponsive toradiation from said source, said detector areas on said one side havingan exposed detector area in each channel always exposed to radiationfrom said source when the source is in said field of view.
 19. Thesystem of claim 17 wherein each of said channels includes two groups ofdetector areas, each detector of each group being adjacent to a detectorof the other group, each detector of each group being separated fromanother detector of that group by a detector of the other group.
 20. Adetector for indicating the angular position of a source of penetratingradiation comprising a detector channel for said radiation having apredetermined field of view, a multiplicity of different, separatedetector areas within said channel each of said areas being susceptibleto exposure to said radiation to derive an output signal in response tosaid radiation impinging thereon, different ones of said areas beingirradiated by said source for differing positions of said sourcerelative to said channel, and shield means for Said radiation positionedbetween said source and detector areas, said shield means having an edgepositioned to define a demarcation line for irradiated and nonirradiateddetector areas of said channel determined by the position of saidsource, all of the detector areas on one side of said line beingirradiated by said source and all of the detector areas on the otherside of said line being shielded from said source by said shield means.21. The system of claim 20 wherein said multiplicity of detector areasare separated from each other by a plurality of second detector areas,means responsive to the amount of radiation impinging on each of saidplurality of areas for deriving first and second bilevel signalsindicative of the total radiation impinging on each area being greaterthan a set level within a predetermined time interval, and means forcomparing the sum of the first-named bilevel signal with the sum of thesecond named bilevel signal.
 22. The system of claim 20 wherein adjacentones of said detector areas in said channel are separated from eachother by areas unresponsive to radiation from said source, said detectorareas on said one side having a detector area in said channel alwaysexposed to radiation from said source when the source is in said fieldof view.